1. Field of the Invention
The present invention relates to a process for measuring the distance of an object with respect to an apparatus for measuring this distance and, more particularly, to such a process implemented with a measurement apparatus delivering both a raw measurement of said distance and a raw measurement of the relative speed of said object with respect to the apparatus. The invention also relates to a device for implementing this process.
2. Description of the Related Art
There is known, for example from the book entitled "Introduction to radar systems" by M. I. Skolnik, published in 1962 in the USA by McGraw-Hill, pages 86 et seq., a periodically frequency-modulated continuous-wave radar, more commonly known as an FMCW radar, for measuring the distance and the relative speed of an object. The employing of such a radar for measuring distances and speeds of motor vehicles is also known from German Patent Application DE-A-40 40 572. According to the process employed in the prior art, a carrier frequency is modulated by a triangular signal for example, this modulated signal is emitted continuously and a fraction of this signal is mixed with the signal reflected by the object whose distance and speed are to be measured. Two beat frequencies are thus obtained, corresponding respectively to the rising ramp and to the falling ramp of the signal for modulating the signal emitted. Following each period of modulation, the distance and the speed are derived from the mean and from the difference respectively of the beat frequencies. The raw values of distance and of speed thus obtained are marred by near-gaussian background noise. This background noise introduces an error into the values supplied, of the order of a meter with regard to distances for example. Such an error cannot be regarded as negligible in certain applications, especially when the radar forms part of an anticollision device with which a motor vehicle is equipped. The distance measurements delivered by the radar are therefore filtered so as to rid them of the inaccuracy due to the background noise. The filtering is preferably carried out with the aid of linear adaptive filters of the "Kalman" type, which are based on the discrepancies between a physical system and a model defined by its state variables. Such filters are described for example in the work entitled "Le filtrage et ses applications" [Filtering and its applications], published by Cepadues 1987, authors: M. Labarrere, J. P. Krief, B. Gimonet. Kalman filters are preferred since they scarcely affect the signal processed. On the other hand, they have the drawback of requiring a large amount of calculating power owing to their complexity.